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Old 25th July 2012, 04:17 AM   #1
xtechee is offline xtechee  United States
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Talking Need for Power Amplifier input low pass

There are two reasons for a low pass filter on the input of an Audio Power Amplifier. The first reason is to attinuate any RF that might get onto the patch cords. The second and most important reason is to slow the input signal rise time to help the amplifier to not slew limmit.
The preamp usually has a faster rise time than the power amp.
If you don't limmit the rise time with an input filter, the amplifier will
sound harsh, as the amplifier output tries to follow the input, but cannot.
The bad sound usually occurs in the complex loud parts of the music.
The sound of a flute or violin will be fine. The rise time of a power amp should be limmited to 2us. 2us seems to be the best compromise.
I test a power amp at 1 to 10 watt out into a .22us orange drop capacitor and 8 ohm load across the output. The output transistors will sing, the watt meter on your variac will peg, but the rise should be nice and rounded with no overshoot or ringing. Once you build your output stage, and have it cast in stone, play with the emmiter current on your diff pair. As you increase the current, the rise time will be faster. The draw back is the power dissipation of
the diff pair and drivers. Don't let them short. More next time.:
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Old 25th July 2012, 08:47 AM   #2
DF96 is offline DF96  England
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Quote:
There are two reasons for a low pass filter on the input of an Audio Power Amplifier. The first reason is to attinuate any RF that might get onto the patch cords. The second and most important reason is to slow the input signal rise time to help the amplifier to not slew limmit.
The preamp usually has a faster rise time than the power amp.
But your signal source probably has a slower rise time than either, unless it is a phono preamp with inadequate equalisation. Keeping out RF is the main benefit. Allowing meaningful square-wave testing is an extra.

PS don't forget that rise time and HF frequency rolloff are not the same thing, although related.

Last edited by DF96; 25th July 2012 at 08:50 AM. Reason: add PS
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Old 25th July 2012, 11:26 AM   #3
Bonsai is offline Bonsai  Taiwan
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Although many modern power amps have slew rates upwards of 100 V/us, you still have to be aware of rise time constraints in order to prevent SID. The input stage overload voltage is set by the LTP degeneration, tail current and the slew rate. On my website, I have a write up about my e-Amp that discusses these issues Ovation e-Amp
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